If you're using Eagle CAD, you need to draw a polygon around the perimeter of your board, and rename it to GND. Make sure you change the 'isolation' tab to around 12 mils or so (this changes how much clearance you want between the ground plane and other traces). Do this on both sides, then hit the rat's nest tool to see it work! I'm not sure how to do this on other PCB layout programs though.

Jeremy Blum has a nice tutorial on youtube (for Eagle) - watch part 2 and skip forward to where he adds the ground plane to see!